1. Field of the Invention
The present invention relates to a thin-film transistor (TFT) suitable for pixel display switching elements in an active matrix display scheme and to a method of producing the same.
2. Description of the Related Art
FIG. 1 is a cross sectional view illustrating the structure of a bottom gate type thin-film transistor. In fabrication of the thin film transistor, a gate electrode 2 of a high-melting point metal such as tungsten or chromium is formed on the surface of an insulating transparent substrate 1. Both sides of the gate electrode 2 are externally tapered on the transparent substrate 1. A silicon oxide film 4 is deposited via a silicon nitride film 3 on the transparent substrate 1 on which the gate electrode 2 is disposed. The silicon nitride film 3 blocks impurities contained in the transparent substrate 1 from intruding into an active region (to be described later). The silicon oxide film 4 works as a gate insulating film. A poly crystalline silicon film 5 is stacked on the silicon oxide film 4 to cross the gate electrode 2. The polycrystalline silicon film 5 acts as an active region.
A stopper 6 of an insulating material such as silicon oxide is formed on the polycrystalline silicon film 5. The polycrystalline silicon film 5 covered with the stopper 6 acts as a channel region 5c while the remaining polycrystalline silicon films 5 respectively act as a source region 5s and a drain region 5d. The silicon oxide film 7 and the silicon nitride film 8 are stacked on the polycrystalline silicon film 5 on which the stopper 6 is formed. Both the silicon oxide film 7 and the silicon nitride film 8 act as an interlayer insulating film for protecting the polycrystalline silicon film 5 including the source region 5c and the drain region 5d and for dispositing the drain line.
A contact hole 9 is formed in a predetermined position of the silicon oxide film 7 and the silicon nitride film 8 formed over the source region 5s while a contact hole 9 is formed in a predetermined position of the silicon oxide film 7 and the silicon nitride film 8 formed over the source region 5d. A source electrode 10s to be connected to the source region 5s is formed in the contact hole 9 while a source electrode 10d to be connected to the drain region 5d is formed in the contact hole 9. An acrylic resin layer 11 transparent to visible rays is stacked over the silicon nitride film 8 in which the source electrode 10s and the drain electrode 10d are formed. The rough surface caused by the gate electrode 2 and the stopper 6 is buried with the acrylic resin layer 11 to become a flat surface.
A contact hole 12 is formed in the acrylic resin film 11 on the source electrode 10s. An ITO (Indium Tin Oxide) transparent electrode 13 is connected to the source electrode 10s via the contact hole 12 and extends toward the surface of the acrylic resin layer 11. The transparent electrode 13 acts as a pixel electrode for the liquid crystal display panel.
A plurality of the above-mentioned thin-film transistors are disposed in a matrix form on the transparent substrate 1, together with pixel electrodes 13. Image data supplied to the drain electrodes 10d is input to the pixel electrodes in response to scanning control signals applied to the gate electrodes 2.
The polycrystalline silicon film 5 is preferably formed with polycrystalline silicon of sufficiently large grain size to act as an active region of a thin-film transistor. The excimer laser annealing method is known as a method of increasing the grain size of the polycrystalline silicon film 5. In this laser annealing method, an amorphous silicon is deposited on the silicon oxide film 4 acting as a gate insulating film. Hydrogen contained in the amorphous silicon film is expelled through a heating process at low temperatures. Then the silicon is temporarily melted by irradiating the excimer laser and is then recrystallized. Since a desired portion on the transparent substrate 1 is locally heated to a high temperature using the laser annealing method, a glass with a low melting point can be used as the transparent substrate 1.
Since the polycrystalline silicon film 5 crystallized through the laser annealing method has many crystalline defects, electrons moving therein tend to be easily trapped. Hence, the crystallized silicon is not desirable as the active region in the transistor. In order to solve such problems, an insulating film containing a great number of hydrogen atoms is formed on the polycrystalline silicon layer 5 temporarily formed. The crystalline defects are then buried with hydrogen atoms by annealing the insulating film in a nitrogen atmosphere.
A silicon nitride film is known as an insulating film containing a large number of hydrogen atoms. The hydrogen atom concentration of a silicon nitride film formed through the plasma CVD method is normally order of 10.sup.22 /cm.sup.3 and is larger by two digits, compared with the hydrogen atom concentration (order of 10.sup.20 /cm.sup.3) in the silicon oxide film formed through the plasma CVD method. For that reason, silicon nitride films are used as a hydrogen atom supply source.
Generally, since the silicon nitride film formed on the active region deteriorates the characteristics of a transistor, a silicon oxide film is formed between the active region and the silicon nitride film as shown in FIG. 1. However, the silicon oxide film 7 of a critical film thickness lying between the polycrystalline silicon film 5 and the silicon nitride film 8 may not supply sufficient hydrogen atoms into the polycrystalline silicon film 5. This problem means that a high-temperature annealing process or prolonged annealing time are required in fabrication steps, thus resulting in a decrease in productivity.